The present invention relates to a rack guide in a rack and pinion type steering system, which supports a rack bar, and more specifically to the rack guide whose sliding frictional resistance against the rack bar is reduced.
Conventionally, a rack and pinion type steering system comprises a pinion provided at an end of a steering shaft and a rack bar having rack teeth which engage with the pinion through which rotation of the steering shaft or the pinion is transformed into reciprocating motion of the rack bar.
In such rack and pinion type steering system, the pinion is rotatably supported in a housing through bearings, while the rack bar is supported in such a manner that a hemi-cylindrical back surface thereof opposite to the teethside is slidable on a rack guide disposed in the housing. The rack guide is formed by mounting a slide bearing member on a supporting base. The slide bearing member usually comprises a backing metal having an arc shape and a synthetic resin layer coated on the backing metal as an inner surface layer so as to support the rack bar on the surface of the synthetic resin layer.
High sliding frictional resistance between the rack guide and the rack bar makes the efficiency of the steering system to be deteriorated, so as to affect on the steering characteristics. To cope with this, it is proposed, for example, in JP-Y2-1-27984 to reduce the contact area between a rack bar and a synthetic resin layer of a slide bearing member in order to reduce the sliding frictional resistance between a rack guide and the rack bar.
According to a specific structure of JP-Y2-127984 for reducing the contact area between the rack bar and the slide bearing member, as shown in the attached drawing of FIG. 6, the rack bar 2 is partially in contact with a synthetic resin layer 1 of the slide bearing member at two restricted linear zones of the surface of the synthetic resin layer 1. The linear zones are located within two surface sections, respectively, which are defined by a symmetrical center line corresponding to the curvature center (line) of the hemi-cylindrical back surface of the rack bar 2 being received in the slide bearing member, the symmetrical center line dividing the hemi-cylindrical back surface of the rack bar 2 to the two surface sections 3 and 4. In FIG. 6, a reference character "LS" means a phantom line which includes the curvature center "O" of the hemi-cylindrical back surface of the rack bar 2 and the above symmetrical center line on the surface of the synthetic resin layer 1. The surface of the synthetic resin layer 1 consists of the two cylindrical surface sections 3 and 4 which have curvature centers "Oa" and "Ob", respectively. The curvature centers "Oa" and "Ob" are located above the curvature center "O" in FIG. 6. Thus, the radii of curvature "Ra" and "Rb" of the two cylindrical surface sections 3 and 4 are greater than that of the radius of curvature "Ro" of the rack bar.
In such structure, however, the hemi-cylindrical back surface of the rack bar 2 is almost in linear contact with the synthetic resin layer 1 at the respective two surface sections 3 and 4. This means that the synthetic resin layer 1 bears the rack bar 2 by small areas. Thus, the bearing load per unit area is considerably large resulting in that the slide bearing member has an inferior durability. Further, the synthetic resin layer 1 is elastically deformed during supporting the rack bar 2 so as to be dented in a circular-arc form along the respective two linear zones being in contact with the hemi-cylindrical back surface of the rack bar 2. Such deformation amount is different from one steering system to another due to variance errors in size of components, the variance errors occurring in assembling the steering systems, and so on. Consequently, the contact area between the synthetic resin layer 1 and the hemi-cylindrical back surface of the rack bar 2 varies from one steering system to another. Thus, the sliding frictional resistance between the synthetic resin layer 1 and the rack bar 2 is different from one steering system to another resulting in a problem that the fabrication quality is unstable in producing such steering systems.